Coefficient of R‐R interval variations under deep breathing load in patients with wild‐type transthyretin amyloid cardiomyopathy: A case‐control study

Abstract Background and Aims An autonomic nervous disorder is an important characteristic of cardiac amyloidosis; however, the prevalence of autonomic dysfunction in wild‐type transthyretin amyloidosis (ATTRwt) has not been established. Analysis of the R‐R interval coefficient of variation (CVR‐R) is a noninvasive method to measure parasympathetic activity. We aimed to assess autonomic dysfunction of ATTRwt and determine the utility of CVR‐R for the detection of ATTRwt in other cardiac diseases. Methods This is a single‐center, retrospective, case‐control study. Fifty patients with heart failure (HF) were studied. The etiologies of HF were as follows: ATTRwt, n = 10; previous myocardial infarction (MI), n = 20; and left ventricular hypertrophy (LVH) due to other disease processes (e.g., aortic stenosis), n = 20. We measured the CVR‐R at rest (CVR‐Rrest), CVR‐R with deep breaths (CVR‐Rbreath), and the change rate (CVR‐Rdiff rate). The relative change formula is as follows: CVR‐Rdiff rate = (CVR‐Rbreath − CVR‐Rrest)/CVR‐Rrest × 100 (%). Results There was no difference in the CVR‐Rrest levels among the three groups. The CVR‐Rdiff rate levels in the ATTRwt group were significantly lower (ATTRwt: −8.77 [−43.8 to 10.9]; LVH: 67.4 [38.7 to 89.4]; MI: 83.7 [60.4 to 142.9]). Based on the receiver operative characteristic curve analysis to identify ATTRwt in HF, the best cut‐off value for the CVR‐Rdiff rate was 19.7 (area under the curve: 0.848). Conclusion Our data suggested autonomic dysfunction in patients with ATTRwt. Measurement of the CVR‐R in HF patients may be a convenient support tool for the detection of ATTRwt.


| INTRODUCTION
Cardiac amyloidosis (CA) is a progressive disease caused by myocardial deposition of amyloid fibrils. 1 There are many different types of amyloidosis, but CA is mainly divided into the following three types: immunoglobulin light chain amyloidosis (AL amyloidosis); hereditary transthyretin amyloidosis (ATTR v ); and wild-type transthyretin amyloidosis (ATTR wt ). The major findings of CA are left ventricular hypertrophy (LVH) and a cardiac conduction disorder. An autonomic nervous system disorder is one of the important features. 2 Heart rate variability (HRV) is a widely used parameter to assess autonomic function. 3 Indeed, HRV has been reported to be a prognostic indicator for CA. [4][5][6] Autonomic dysfunction is frequently observed in ATTR v and AL amyloidosis. Reduction of the standard deviation of beat-to-beat or NN intervals (SDNN) ≤ 50 ms is a strong predictor of 1-year mortality in patients with AL amyloidosis 4 ; however, the prevalence of autonomic dysfunction and the usefulness of HRV in patients with ATTR wt has not been established. The coefficient of variation in R-R intervals (CVR-R) is a measure of HRV that mainly reflects the parasympathetic nervous system. The aims of this study were: (1) to assess autonomic dysfunction of ATTR wt using the CVR-R and (2) to determine the utility of the CVR-R for detecting ATTR wt in patients with other cardiac conditions of LVH or myocardial infarction. Patients with arrhythmias (e.g., atrial fibrillation, ventricular premature contractions, II-III atrioventricular block, paced rhythm, and a history of catheter ablation of cardiac arrhythmias) were excluded. We also excluded patients with NYHA classification IV, AL amyloidosis, AA amyloidosis, ATTR v , and a history of a MI that occurred 1 year before enrollment. In the absence of a genetic diagnosis, ATTR v was ruled out based on the family medical history. A small number of registrations was expected because of the rarity of the disease, strict eligibility criteria, and the single-center study.

| METHODS
Study sample size was referenced from previous studies. 5,6 Blood samples were collected from patients who were clinically stable. Serum cardiac troponin I and plasma brain natriuretic peptide After at least 5 min at rest, a standard 12-lead ECG was recorded in the spine position (VS-3000E; Fukuda Denshi), and the R-R interval was analyzed for 1 min. Drugs with anticholinergic effects were discontinued before testing. Following measurement of the R-R interval with normal breathing, the R-R interval with deep breaths at a rate of six times/min was analyzed. Based on the mean R-R interval (mRR) and the R-R standard deviation (RR-SD), the CVR-R was calculated as follows: RR-SD/mRR × 100(% Following measurement of the CVR-R at rest (CVR-R rest ) and the CVR-R with deep breathing (CVR-R breath ), the difference rate (CVR-R diff rate ) was calculated using the following formula:  Figure 2). Based on the ROC analysis the best CVR-R diff rate cut-off value with which to identify ATTR wt in HF was 19.7 (AUC: 0.848). Furthermore, at this cut-off value the sensitivity and specificity for predicting ATTR wt were 97.5% and 80%, respectively. There were no statistically differences in the AUC values between CVR-R diff rate and troponin I.

Analysis of interplay between CVR-R values and MRI could not be
performed due to the small number. There was no significant correlation between CVR-R and LVMI (data not shown).

| DISCUSSION
The validity of autonomic function test in the pathological assessment of ATTR v has been established. 16 As for ATTR wt , on the other hand, the data is still scarce. To our best knowledge, this study is the first report about the usefulness of deep breath induced CVR-R fluctuation in the patients with ATTR wt . Diagnostic delay in ATTR wt not only precludes adequate therapy, including tafamidis, but also increases healthcare costs. 17 This method could be one of the clues for early diagnosis of ATTR wt .
Measurement of the CVR-R is a quantitative procedure to evaluate cardiac parasympathetic activation. 18,19 The CVR-R is routinely used in Japan to assess the cardiac ANS in patients with diabetes. 20 The CVR-R values are associated with glycemic variability, 21 anemia, 22 renal function, 23

ACKNOWLEDGMENT
We thank the Amakusa Medical Center staff members, including the physicians, comedical staff, pharmacists, and office employees.

CONFLICT OF INTEREST
The authors declare no conflict of interest.

DATA AVAILABILITY STATEMENT
Participants of this study did not agree for their data to be shared publicly, so supporting data are not available.

ETHICS STATEMENT
All procedures were performed in accordance with the Declaration of Helsinki and its amendments. The study protocol was approved by the Institutional Review Board of the Amakusa Medical Center (approval no. 20210316-5).

TRANSPARENCY STATEMENT
The lead author Yasuhiro Nagayoshi affirms that this manuscript is an honest, accurate, and transparent account of the study being reported; that no important aspects of the study have been omitted; and that any discrepancies from the study as planned (and, if relevant, registered) have been explained.